Co-catalyzed synthesis of 1,4-dicarbonyl compounds using TBHP oxidant

Article abstract of DOI:10.1021/ol5004687

A Co-catalyzed reaction for the construction of 1,4-dicarbonyls has been reported in which cascade organocobalt addition/trapping/Kornblum-DeLaMare rearrangement were involved. In view of the easy availability of starting materials, wide substrate scope, high functionality tolerance, and operational simplicity, this protocol constituted a simple, practical, and powerful alternative compared with previous approaches.

Full text of DOI:10.1021/ol5004687

Letter
pubs.acs.org/OrgLett
Co-Catalyzed Synthesis of 1,4-Dicarbonyl Compounds Using TBHP
Oxidant
Feng Zhang, Peng Du, Jijun Chen, Hongxiang Wang, Qiang Luo, and Xiaobing Wan*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: A Co-catalyzed reaction for the construction of
1,4-dicarbonyls has been reported in which cascade organo-
cobalt addition/trapping/Kornblum−DeLaMare rearrange-
ment were involved. In view of the easy availability of starting
materials, wide substrate scope, high functionality tolerance, and operational simplicity, this protocol constituted a simple,
practical, and powerful alternative compared with previous approaches.
Scheme 1. Our Strategy for the Synthesis of 1,4-Dicarbonyls
1,4-Dicarbonyl compounds are versatile building blocks for the
synthesis of various carbocyclic and heterocyclic compounds,1
such as cyclopentenones, furan, thiophene, pyridazines, and
pyrrole derivatives. As a result, significant efforts have been
directed toward the synthesis of this highly valuable synthon.
For example, the conjugate addition of acyl anion equivalents to
Michael acceptors provided a powerful method for 1,4-
dicarbonyl synthesis.2 Other attractive approaches consist of
nucleophilic substitution of α-haloketones,3 chain extension of
1,3-dicarbonyl compounds,4 oxidative coupling of enolates5 or
alkenes,6 and the addition of homoenolate equivalents to acid
derivatives.7 Recently, enolate heterocoupling has also emerged
as a straightforward approach for the chemoselective formation
of 1,4-dicarbonyl.8 Despite the great successes achieved in this
field, significant improvement is still in demand in terms of
availability of starting materials, substrate scope, and functional
group tolerance.
tBuOO• radical in high chemoselectivity without the influence
of competitive protonolysis10d,e,k,m (Scheme 1, path b) and β-H
elimination9c,e−g,i,10b,c,g,l (Scheme 1, path c), both of which
were well documented to be side reactions. Another noticeable
side reaction is the oxidation of styrene in the presence of
TBHP through oxidative cleavage,13 epoxidation,14 or Wacker
oxidation15 (Scheme 1, path d).
We began our condition optimization with the use of Et3N as
the solvent, as it could promote Kornblum−DeLaMare
rearrangement and, meanwhile, suppress proton abstraction.16
A high yield of 86% was obtained by treating the reaction of 1-
(tert-butyl)-4-vinylbenzene and ethyl 2-bromopropanoate with
10 mol % Co(acac)2 and 4.0 equiv of TBHP at 100 °C for 24 h
under an air atmosphere (Table S1, entry 1; see Supporting
Information). Co-catalyzed reductive coupling has been well
developed in the past decades.9e−h Herein, we demonstrate a
rare example of Co-catalyzed oxidative coupling,17 in which
both a C−C single bond and CO double bond were
constructed selectively in one pot. Remarkably, a similar result
was obtained under an inert atmosphere, implying that
molecular oxygen did not act as an oxidant for this
Current methods for 1,4-dicarbonyl synthesis have mainly
relied on the reactions between two derivatives of carbonyl
compounds. The cobalt-mediated radical reaction9,10 has
emerged as an attractive and powerful tool in the domain of
synthetic organic chemistry. Recently, we developed several
radical reactions using TBHP as an oxidant,11a−h in which the
tBuOO• radical was generated in situ and served as a key
intermediate. Combining the above two concepts, tandem
addition of organocobalt to styrenes/interception by the
tBuOO• radical was envisioned to afford a tert-butylperoxide
compound, which could undergo Kornblum−DeLaMare
rearrangement12 to form the expected 1,4-dicarbonyl product
(Scheme 1, path a). To the best of our knowledge, both the
t
trapping of organocobalt with the BuOO• radical and the
combination of organocobalt chemistry and Kornblum−
DeLaMare rearrangement are so far underexplored. From the
synthetic point of view, this methodology is also intriguing due
to its direct use of easily available styrenes and α-
bromocarbonyls.
Many obstacles to the formation of 1,4-dicarbonyls remain in
the hypothesis. One is how to trap the benzylcobalt by the
Received: February 13, 2014
Published: March 17, 2014
© 2014 American Chemical Society
1932
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Letter
a
transformation. This is in sharp contrast with the reported
synthesis of 1,4-dicarbonyl via a radical process6 (Table S1,
entry 20).
Scheme 4. Scope of α-Bromoamides
With the optimized reaction conditions in hand, we next
evaluated a wide range of substituted styrenes. As shown in
Scheme 2, the reaction demonstrated a broad tolerance toward
a
Scheme 2. Scope of Styrenes
a
1.5 mmol of 1a, 0.5 mmol of 2, 0.05 mol of Co(acac)2, 2.0 mmol of
TBHP, 2.0 mL of Et3N, at 100 °C for 24 h in a sealed tube.
An array of α-bromoketones were also investigated as
starting materials. As shown in Scheme 5, yields for these
a
a
Scheme 5. Scope of α-Bromoketones
0.5 mmol of 1, 1.5 mmol of 2a, 0.05 mol of Co(acac)2, 2.0 mmol of
TBHP, 2.0 mL of Et3N, at 100 °C for 24 h in a sealed tube.
a variety of functional groups such as halide (3c−3f, 3l), nitrile
(3j), ether (3h, 3m), benzylic C−H bond (3g), and BocNH
(3i). Substitution at all ortho (3m), meta (3f), and para (3c−
3e, 3g−3j) positions are compatible. Heterocyclics such as 4-
vinylpyridine and 2-vinylthiophene are suitable substrates,
albeit resulting in slightly lower yields (products 3k and 3l).
When aliphatic alkenes were subjected to the reaction, no
significant amount of the corresponding 1,4-dicarbonyls were
obtained under standard conditions.
With 4-tert-butylstyrene (1a) as the coupling partner, α-
bromoesters carrying different substituents all reacted smoothly
generating the desired products in moderate to high yields
(Scheme 3). Notably, γ-butyrolactone was also apt, furnishing
4f in 60% yield.
a
0.5 mmol of 1a, 1.5 mmol of 2, 0.05 mol of Co(acac)2, 2.0 mmol
TBHP, 2.0 mL of Et3N, at 100 °C for 24 h in a sealed tube.
Apart from α-bromoesters, α-bromoamides are also exam-
ined (Scheme 4). Broad functionality tolerance was once again
observed for various substituents including benzyl (5c), ether
(5f, 5j), amine (5g), and even an active HNCOPh group (5k),
to give moderate to high yields. Either cyclic or acyclic amides
are compatible with the reaction conditions.
transformations varied from 49% to 75%. Cyclic (6a, 6c),
acyclic (6b, 6e), and aromatic ketones (6d) could all be
accepted. It is noteworthy that α-bromonitrile also proved to be
a compatible reaction partner, leading to the synthetically useful
β-nitrile ketone 6f in moderate yield.
A reaction of 4-tert-butylstyrene and 2-bromo-N,N-diphenyl-
acetamide under optimized conditions was performed for 0.5 h.
Trace amounts of byproducts 7 and 8 from protonolysis and a
β-H elimination reaction of a benzylcobalt intermediate were
found in the crude product by means of LC-MS (Figure 1).
When the radical trap BHT (3,5-di-tert-butyl-4-hydroxy-
toluene) was added, the corresponding adducts 9 and 10
were observed in this reaction mixture. The detection of 9 and
a
Scheme 3. Scope of α-Bromoesters
a
0.5 mmol of 1a, 1.5 mmol of 2, 0.05 mol of Co(acac)2, 2.0 mmol of
TBHP, 2.0 mL of Et3N, at 100 °C for 24 h in a sealed tube.
Figure 1. Investigation of reaction mechanism.
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Letter
10 provided the proof of homolysis of the carbon−cobalt bond
of the organocobalt intermediate (see path a in Scheme 1) in
this oxidative process.
organocobalt complexes cannot be ruled out in this trans-
formation.
In summary, we have implemented a new termination mode
t
The above experiments provide evidence for the formation of
organocobalt intermediates and their thermal homolysis to
generate radicals during this 1,4-dicarbonyl formation reaction.
Next, we try to investigate the role of tert-butylperoxide in this
transformation. After careful examination, tert-butylperoxide 11
was observed in the reaction mixture. As further evidence, it
could undergo Kornblum−DeLaMare rearrangement to afford
the predicted γ-ketone amide 5h in 61% yield (eq 1), implying
of the carbon−cobalt bond by trapping with the BuOO•
radical, wherein the in situ generated tert-butylperoxide can
then undergo Kornblum−DeLaMare rearrangement to furnish
a range of 1,4-dicarbonyls. Our method utilizes easily available
starting materials, offers operational simplicity, and enjoys a
broad substrate scope as well as functionality tolerance.
Currently, interception of an organocobalt by other radicals
to develop a new methodology is in progress in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and full spectroscopic data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
it functions as a reaction intermediate in this transformation. It
is worth noting that this result was different form Lei’ report,18
in which β-peroxy ketone was not directly involved in the
generation of the desired α,β-unsaturated ketone. Meanwhile, a
very poor yield of 5h was achieved when benzyl alcohol 12 was
subjected to the same reaction system (eq 2), thereby ruling
out the possibility of the latter’s direct oxidation into the 1,4-
dicarbonyl product.19a−c
*E-mail: wanxb@suda.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Natural Science Foundation of
China (No. 21272165).
Based on the above-mentioned findings, a plausible
mechanism is outlined in Scheme 6. The cobalt catalyst is
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t
t
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